The belief that all toilets in the Northern Hemisphere flush counter-clockwise while those in the Southern Hemisphere flush clockwise is entirely false. However, the premise behind the myth is actually true. The Coriolis effect refers to the way the Earth’s constant counterclockwise rotation influences the trajectory of moving objects.

As the diameter of the Earth is so much greater at the equator – 40,076 kilometres (24,902 miles) – than at the North and South Poles – 0 kilometres (0 miles) – the land is moving much faster there. Therefore, if an object travels through the air between the equator and North Pole, it will deflect to the right because it maintains the momentum of its place of origin, while an object travelling between the equator and the South Pole will veer to the left for the same reason.

This doesn’t affect your toilet’s flush because the Earth doesn’t spin fast enough to adjust the trajectory of such a small flow of water. Instead, the direction in which the water drains is actually down to the design of the toilet bowl and how the flush angles the stream of water.

However, the Coriolis effect does have an effect on much larger bodies of water, such as the oceans, as well as air currents that dictate the direction of hurricane winds. In fact, without the Coriolis effect, we wouldn’t have westerly or easterly winds at all, as the wind would simple flow from the North and South Poles toward the equator and back again without anything to deflect it.

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